In discrete-time interpolation of sample values of a sampled signal, the sample values must represent a band-limited signal or be band-limited by an appropriate digital filter. By means of the interpolation, the sampling rate is to be changed by an arbitrary numerical ratio, for example, or an intermediate signal value is to be calculated while retaining the respective sampling rate. Such applications are in the processing of digital video and audio signals, particularly in digital television receivers, video recorders, audio reproduction equipment, or computer-based or television-set-based multimedia processing equipment.
In the simplest case, such an interpolation is a linear interpolation, which starts from adjacent given values of the sampling sequence. This simple interpolation has a disadvantage in that it suppresses signal frequencies equal to half the sampling rate. In the case of vido signals, this reduces the picture sharpness, and may even cause clearly visible picture disturbances if such suppression results in interferences in the case of periodic picture contents.
U.S. Pat. No. 4,760,542, issued Jul. 26, 1988, incorporated by reference herein, discloses an interpolator for digital signals which circumvents part of these disadvantages by being implemented as a digital filter combination which contains, in the direction of signal flow, a discrete-time third-order interpolation filter and a linear interpolation filter. The discrete-time interpolation filter forms, in addition to the existing sample values, an intermediate value exactly in the middle between two sampling instants. Through the discrete-time filter, the sampling rate is effectively doubled. In the case of the discrete-time filter described, the frequency response in the passband for higher frequencies can be changed by increasing or decreasing particular frequency components by means of a weighting factor. Using a delay chain consisting of three delay stages and a logical combination of the differently delayed sample values, the individual frequency components are formed, which may be weighted differently by means of multipliers and are finally combined. By linear interpolation, the interpolated value is then calculated from the two closest secondary sample values of the new sampling sequence at the desired interpolating instant. To perform the linear interpolation, only a subtracter, a multiplier, and an adder are needed in the example given, with the mixture ratio of the two sample values being controlled by the multiplier and an applied multiplication factor. The multiplication factor corresponds to an interpolating instant normalized to the secondary sampling rate.
The filter function of this filter combination, however, does not meet the increased requirements placed on digital interpolation. It is therefore an object of the invention to provide an improved digital filter for performing an arbitrary temporal interpolation of a sampled signal which permits improved picture reproduction, particularly with digitized video signals.